A 'four-eyed' fish from the deep-sea: Bathylychnops exilis Cohen, 1958

Diversity and evolution of optically complex eyes in a family of deep-sea fish: Ocular diverticula in barreleye spookfish (Opisthoproctidae)

Several families of mesopelagic fish have tubular eyes that are usually upwardly directed. These maximise sensitivity to dim downwelling sunlight and dorsal bioluminescence, as well as facilitating the detection of dark silhouettes above the animal. Such eyes, however, have a much-reduced field of view and will not be sensitive to, for example, lateral and ventral bioluminescent stimuli. All mesopelagic Opisthoproctidae so far examined have evolved mechanisms for extending the limited visual field of their eyes using approximately ventrolaterally directed, light-sensitive, diverticula. Some genera have small rudimentary lateral retinal areas capable of detecting only unfocussed illumination. Others have more extensive structures resulting in eyes that simultaneously focus light from above onto the main retina of the tubular eye using a lens, while diverticula produce focussed images of ventrolateral illumination using either reflection or possibly refraction. These bipartite structures represent perhaps the most optically complex of all vertebrate eyes. Here we extend the limited previous data on the ocular morphology of five Opisthoproctidae (Opisthoproctus soleatus, Winteria telescopa, Dolichopteryx longipes, Rhynchohyalus natalensis, and Bathylychnops exilis) using a combination of histology and magnetic resonance imaging and provide a preliminary description of the eyes of Macropinna microstoma. We note an increase in diverticular complexity over the life span of some species and quantify the contribution of the diverticulum to the eye’s total neural output in D. longipes and R. natalensis (25 and 20%, respectively). To help understand the evolution of Opisthoproctidae ocular diversity, a phylogeny, including all the species whose eye types are known, was reconstructed using DNA sequences from 15 mitochondrial and four nuclear genes. Mapping the different types of diverticula onto this phylogeny suggests a process of repeated evolution of complex ocular morphology from more rudimentary diverticula.

Reflecting optics in the diverticular eye of a deep-sea barreleye fish (Rhynchohyalus natalensis)

We describe the bi-directed eyes of a mesopelagic teleost fish, Rhynchohyalus natalensis, that possesses an extensive lateral diverticulum to each tubular eye. Each diverticulum contains a mirror that focuses light from the ventro-lateral visual field. This species can thereby visualize both downwelling sunlight and bioluminescence over a wide field of view. Modelling shows that the mirror is very likely to be capable of producing a bright, well focused image. After Dolichopteryx longipes, this is only the second description of an eye in a vertebrate having both reflective and refractive optics. Although superficially similar, the optics of the diverticular eyes of these two species of fish differ in some important respects. Firstly, the reflective crystals in the D. longipes mirror are derived from a tapetum within the retinal pigment epithelium, whereas in R. natalensis they develop from the choroidal argentea. Secondly, in D. longipes the angle of the reflective crystals varies depending on their position within the mirror, forming a Fresnel-type reflector, but in R. natalensis the crystals are orientated almost parallel to the mirror's surface and image formation is dependent on the gross morphology of the diverticular mirror. Two remarkably different developmental solutions have thus evolved in these two closely related species of opisthoproctid teleosts to extend the restricted visual field of a tubular eye and provide a well-focused image with reflective optics.

Vision in the deep sea

The deep sea is the largest habitat on earth. Its three great faunal environments--the twilight mesopelagic zone, the dark bathypelagic zone and the vast flat expanses of the benthic habitat--are home to a rich fauna of vertebrates and invertebrates. In the mesopelagic zone (150-1000 m), the down-welling daylight creates an extended scene that becomes increasingly dimmer and bluer with depth. The available daylight also originates increasingly from vertically above, and bioluminescent point-source flashes, well contrasted against the dim background daylight, become increasingly visible. In the bathypelagic zone below 1000 m no daylight remains, and the scene becomes entirely dominated by point-like bioluminescence. This changing nature of visual scenes with depth--from extended source to point source--has had a profound effect on the designs of deep-sea eyes, both optically and neurally, a fact that until recently was not fully appreciated. Recent measurements of the sensitivity and spatial resolution of deep-sea eyes--particularly from the camera eyes of fishes and cephalopods and the compound eyes of crustaceans--reveal that ocular designs are well matched to the nature of the visual scene at any given depth. This match between eye design and visual scene is the subject of this review. The greatest variation in eye design is found in the mesopelagic zone, where dim down-welling daylight and bio-luminescent point sources may be visible simultaneously. Some mesopelagic eyes rely on spatial and temporal summation to increase sensitivity to a dim extended scene, while others sacrifice this sensitivity to localise pinpoints of bright bioluminescence. Yet other eyes have retinal regions separately specialised for each type of light. In the bathypelagic zone, eyes generally get smaller and therefore less sensitive to point sources with increasing depth. In fishes, this insensitivity, combined with surprisingly high spatial resolution, is very well adapted to the detection and localisation of point-source bioluminescence at ecologically meaningful distances. At all depths, the eyes of animals active on and over the nutrient-rich sea floor are generally larger than the eyes of pelagic species. In fishes, the retinal ganglion cells are also frequently arranged in a horizontal visual streak, an adaptation for viewing the wide flat horizon of the sea floor, and all animals living there. These and many other aspects of light and vision in the deep sea are reviewed in support of the following conclusion: it is not only the intensity of light at different depths, but also its distribution in space, which has been a major force in the evolution of deep-sea vision.